Various tools used in the drilling of wellbores (i.e., downhole tools) include a spring as an operative component of the tool. A mechanical jar is one such tool, and is used to free equipment stuck in a borehole by delivery of an upward or downward axial blow. A jar typically includes an inner mandrel and an outer housing telescopically coupled together for relative axial sliding movement. The mandrel carries a hammer and the housing carries an anvil By directing the hammer to impact the anvil at high velocity a substantial jarring force is imparted to stuck equipment, which is often sufficient to free the stuck equipment.
A typical mechanical jar includes a mechanical latch set to release at a pre-determined force. The hammer of the jar will remain securely locked in position until a force exceeding the latch setting is applied to the jar (e.g., by operation of the drawworks on the drill string). Thus, the latch prevents the jar from firing under normal drilling, tripping, or handling loads.
The latch mechanism of the jar includes an axially aligned stack of springs (e.g., disc or Belleville springs) whose compression may be adjusted by spacers to vary the latch setting. A stack of disc springs included in the latch mechanism will have a total height and associated spring rate that varies due to manufacturing tolerances and spring setting. Consequently, each spring assembly may require a different preload to achieve the precise latch setting that is desired. Conventionally, to set the preload, a number of spacers estimated to produce a desired latch setting are installed with the springs before the jar is completely assembled. The assembled jar is then tested on a jar tester where the tensile and compressive loads required to un-latch the jar are measured. If the latch settings are not as desired, the tool must be moved to a breakout machine, partially disassembled, and the spacers readjusted (i.e., spacers added or removed). Thereafter, the jar is reassembled, moved back to the jar tester and retested. Moving the downhole tool between machines for testing, and assembling and disassembling the tool to achieve the desired latch setting is costly and time consuming.